Solid-state image display devices utilizing light-emissive pixels are well known and widely used. For example, OLED devices are used in flat-panel displays, in both passive- and active-matrix configurations, and in both top-emitter and bottom-emitter designs. Control circuits for OLED displays are also well known in the art and include both voltage- and current-controlled schemes.
Conventional passive-matrix OLED displays employ a variable current in combination with a fixed period during which the OLED light-emitting element emits light. Successive rows or columns of OLED elements are energized and the entire OLED display is refreshed at a rate sufficient to avoid the appearance of flicker. For example, WO2003034389 A2 entitled “System and Method for Providing Pulse Amplitude Modulation for OLED Display Drivers” published Apr. 24, 2003 describes a pulse width modulation driver for an organic light emitting diode display. One embodiment of a video display comprises a voltage driver for providing a selected voltage to drive an organic light emitting diode in a video display. The voltage driver may receive voltage information from a correction table that accounts for aging, column resistance, row resistance, and other diode characteristics.
Active-matrix OLED devices suffer from manufacturing variability that leads to non-uniformity in OLED displays. Moreover, the OLED light-emitting elements themselves degrade over time and with use, thereby modifying the light output from the devices in response to control and power signals. Hence, conventional active-matrix drive methods that employ stored charge deposited on a local capacitor at each pixel site to control a drive circuit for driving an OLED light-emitting element, will experience an undesirable variation in light output from element to element.
There are a variety of known schemes for compensating for sources of non-uniformity in an active-matrix OLED display, in particular for variations in drive transistor threshold variation. For example, US20040207614 entitled “Display device of active matrix drive type” describes such a pixel-control circuit. U.S. Pat. No. 6,777,888 entitled “Drive circuit to be used in active matrix type light-emitting element array” describes another such design. However, such circuit designs are typically complex and employ many more elements and control signals. Hence, such an approach may reduce manufacturing yields and reduce the light-emissive area of the OLED device.
Because one source of non-uniformity in an OLED display results from variability in the threshold switching characteristics of thin-film drive transistors employed in active-matrix designs, one approach to improving uniformity in an active-matrix OLED display is to employ pulse-width modulation techniques in contrast to charge-deposition control techniques. These pulse-width modulation techniques operate by driving the OLED at a maximum current and brightness for a specific first amount of time and then turning the OLED off for a second amount of time. If the sum of the first and second amounts of time is sufficiently small, the flicker resulting from turning the OLED on and off periodically will not be perceptible to a viewer. The brightness of the OLED element is then controlled by varying the ratio of amount of time that the OLED is turned on in comparison to the amount of time that the OLED is turned off.
A variety of methods for controlling an OLED display using pulse-width modulation are known. For example, U.S. Pat. No. 6,809,710 entitled “Gray scale pixel driver for electronic display and method of operation therefore” granted Oct. 26, 2004 discloses a circuit for driving an OLED in a graphics display. The circuit employs a current source connected to a terminal of the OLED operating in a switched mode. The current source is responsive to a combination of a selectively set cyclical voltage signal and a cyclical variable amplitude voltage signal. The current source, when switched on, is designed and optimized to supply the OLED with the amount of current necessary for the OLED to achieve maximum luminance. When switched off, the current source blocks the supply of current to the OLED, providing a uniform black level for an OLED display. The apparent luminance of the OLED is controlled by modulating the pulse width of the current supplied to the OLED, thus varying the length of time during which current is supplied to the OLED.
By using a switched mode of operation at the current source, the circuit is able to employ a larger range of voltages to control the luminance values in a current-driven OLED display. However, use of current-driven circuits is complex and requires a large amount of space for each pixel in a display device.
There are also methods known for providing both a pulse width control and a variable charge deposition control in a single circuit. U.S. Pat. No. 6,670,773 entitled “Drive circuit for active matrix light emitting device” suggests a transistor in parallel with an OLED element. The described technique, however, diverts driving current from an OLED, thereby decreasing the operating efficiency of the circuit. Other designs employ circuit elements in series with the OLED element for controlling or measuring the performance of the OLED element. For example, WO2004036536 entitled “Active Matrix Organic Electroluminescent Display Device” published Apr. 29, 2004 illustrates a circuit having additional elements in series with an OLED element. However, when placed in series with an OLED element, transistors will increase the overall voltage necessary to drive the OLED element or may otherwise increase the overall power used by the OLED element or decrease the range of currents available to the OLED element.
An additional problem faced by OLED devices is the change in OLED material characteristics as the OLED elements are used. Typically, the OLED elements become less efficient and have a higher effective resistance. Both of these factors tend to increase the voltage needed to drive current through the OLED element. This increases the overall voltage of the system, inhibiting the brightness of the elements at a given voltage.
Thin-film transistors used to drive a typical active-matrix OLED element also place restrictions on operation. A typical transistor has an operating range defined by its current/voltage characteristics. At low voltages or currents, a transistor will no longer operate in a region with a linear response to changes in control signals. Transistor circuits are designed to operate within a restricted range where the performance of the transistor will behave as desired. If control signals move the transistors out of the restricted operating range, the device will no longer behave as desired.
New OLED materials and structures are under development that greatly reduce the current needed to produce a suitable light output. See, for example, U.S. Patent Publication No. 2003/0170491 by Liang-Sheng L. Liao et al., entitled “Providing an Organic Electroluminescent Device Having Stacked Electroluminescent Units”. These structures require changes in driving circuits to provide suitable control within a desired operating region of a transistor circuit.
There is a need therefore for an improved control circuit for active-matrix OLED devices having simplified design, flexible control, and desired operation that does not increase the power used by the circuit.